A long way gone : memoirs of a boy soldier / Ishmael Beah.

Enhanced descriptions from Syndetics:

In A Long Way Gone , Ishmael Beah tells a riveting story: how at the age of twelve, he fled attacking rebels and wandered a land rendered unrecognizable by violence. By thirteen, he'd been picked up by the government army, and Beah, at heart a gentle boy, found that he was capable of truly terrible acts.

My new friends have begun to suspect I haven't told them the full story of my life.
"Why did you leave Sierra Leone?"
"Because there is a war."
"You mean, you saw people running around with guns and shooting each other?"
"Yes, all the time."
"Cool."
I smile a little.
"You should tell us about it sometime."
"Yes, sometime."

This is how wars are fought now: by children, hopped-up on drugs and wielding AK-47s. Children have become soldiers of choice. In the more than fifty conflicts going on worldwide, it is estimated that there are some 300,000 child soldiers. Ishmael Beah used to be one of them.

What is war like through the eyes of a child soldier? How does one become a killer? How does one stop? Child soldiers have been profiled by journalists, and novelists have struggled to imagine their lives. But until now, there has not been a first-person account from someone who came through this hell and survived.

This is a rare and mesmerizing account, told with real literary force and heartbreaking honesty.